kustomize/examples/kvSourceGoPlugin.md

Generating Secrets

What's a Secret?

Kubernetes ConfigMaps and Secrets are both key:value (KV) maps, but the latter is intended to signal that its values have a sensitive nature - e.g. ssh keys or passwords.

Kubernetes assumes that the values in a Secret are base64 encoded, and decodes them before actual use (as, say, the argument to a container command). The user that creates the Secret must base64 encode the data, or use a tool that does it for them. This encoding doesn't protect the secret from anything other than an over-the-shoulder glance.

Protecting the actual secrecy of a Secret value is up to the cluster operator. They must lock down the cluster (and its etcd data store) as tightly as desired, and likewise protect the bytes that feed into the cluster to ultimately become the content of a Secret value.

Make a place to work

DEMO_HOME=$(mktemp -d)

Secret values from local files

kustomize has three different ways to generate a secret from local files:

• get them from so-called env files (NAME=VALUE, one per line),
• consume the entire contents of a file to make one secret value,
• get literal values from the kustomization file itself.

Here's an example combining all three methods:

Make an env file with some short secrets:

cat <<'EOF' >$DEMO_HOME/foo.env
ROUTER_PASSWORD=admin
DB_PASSWORD=iloveyou
EOF

Make a text file with a long secret:

cat <<'EOF' >$DEMO_HOME/longsecret.txt
Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt
ut labore et dolore magna aliqua.
EOF

And make a kustomization file referring to the above and additionally defining some literal KV pairs:

cat <<'EOF' >$DEMO_HOME/kustomization.yaml
secretGenerator:
- name: mysecrets
  kvSources:
  - name: envfiles
    pluginType: builtin
    args:
    - foo.env
  - name: files
    pluginType: builtin 
    args:
    - longsecret.txt
  - name: literals
    pluginType: builtin
    args:
    - FRUIT=apple
    - VEGETABLE=carrot
EOF

The above syntax is alpha behavior at HEAD, for v2.1+.

The default value of pluginType is builtin, so the pluginType fields could be omitted.

The equivalent v2.0.3 syntax (still supported) is

secretGenerator:
- name: mysecrets
  env: foo.env
  files:
  - longsecret.txt
  literals:
  - FRUIT=apple
  - VEGETABLE=carrot

Now generate the Secret:

result=$(kustomize build $DEMO_HOME)
echo "$result"
# Spot check the result:
test 1 == $(echo "$result" | grep -c "FRUIT: YXBwbGU=")

This emits something like

apiVersion: v1
kind: Secret
metadata:
  name: mysecrets-hfb5df789h
type: Opaque
data:
  FRUIT: YXBwbGU=
  VEGETABLE: Y2Fycm90
  ROUTER_PASSWORD: YWRtaW4=
  DB_PASSWORD: aWxvdmV5b3U=
  longsecret.txt: TG9yZW0gaXBzdW0gZG9sb3Igc2l0I... (elided)

The name of the resource is a prefix, mysecrets (as specfied in the kustomization file), followed by a hash of its contents.

Use your favorite base64 decoder to confirm the raw versions of any of these values.

The problem that these three approaches share is that the purported secrets must live on disk.

This adds additional security questions - who can see the files, who installs them, who deletes them, etc.

Secret values from anywhere

New alpha behavior at HEAD, for v2.1+

A general alternative is to enshrine secret value generation in a Go plugin.

The values can then come in via, say, an authenticated and authorized RPC to a password vault service.

Here's a trivial plugin that provides hardcoded values:

cat <<'EOF' >$DEMO_HOME/kvMaker.go
package main
var database = map[string]string{
  "TREE":      "oak",
  "ROCKET":    "Saturn V",
  "FRUIT":     "apple",
  "VEGETABLE": "carrot",
  "SIMPSON":   "homer",
}

type plugin struct{}
var KVSource plugin
func (p plugin) Get(
    root string, args []string) (map[string]string, error) {
  r := make(map[string]string)
  for _, k := range args {
    v, ok := database[k]
    if ok {
      r[k] = v
    }
  }
  return r, nil
}
EOF

The two crucial items needed to load and query the plugin are

1. the public symbol KVSource,
2. its public Get method and signature.

Plugins that generate KV pairs for kustomize must be installed at

$XDG_CONFIG_HOME/kustomize/plugins/kvSource

XDG_CONFIG_HOME is an environment variable used by many programs as the root of a configuration directory. If unspecified, the default $HOME/.config is used. The rest of the required directory path establishes that the files found there are kustomize plugins for generating KV pairs.

Compile and install the plugin:

kvSources=$DEMO_HOME/kustomize/plugins/kvSources
mkdir -p $kvSources
go build -buildmode plugin \
  -o $kvSources/kvMaker.so \
  $DEMO_HOME/kvMaker.go

Create a new kustomization file referencing this plugin:

cat <<'EOF' >$DEMO_HOME/kustomization.yaml
secretGenerator:
- name: mysecrets
  kvSources:
  - name: kvMaker
    pluginType: go
    args:
    - FRUIT
    - VEGETABLE
EOF

Finally, generate the secret, setting XDG_CONFIG_HOME appropriately:

result=$( \
  XDG_CONFIG_HOME=$DEMO_HOME \
  kustomize \
  --enable_alpha_goplugins_accept_panic_risk \
  build $DEMO_HOME )
echo "$result"
# Spot check the result:
test 1 == $(echo "$result" | grep -c "FRUIT: YXBwbGU=")

This should emit something like:

apiVersion: v1
kind: Secret
metadata:
  name: mysecrets-bdt27dbkd6
type: Opaque
data:
 FRUIT: YXBwbGU=
 VEGETABLE: Y2Fycm90

i.e. a subset of the same values as above.

Go plugins work well, but their usage may fail (the program may crash) if there's too much skew between main program and plugin compilation conditions. For this reason, their use is protected by an annoyingly long opt-in flag (--enable_alpha_goplugins_accept_panic_risk) intended to make the user aware of this risk.

It's safest to use Go plugins in the context of a container image holding both the main and the Go plugins it needs, all built on the same machine, with the same transitive libs and the same compiler version.